FIGS. 1A and 1B show examples of the construction of an optical access network.
FIG. 1A shows an example of the construction of an optical access network of a point-to-point access (one-to-one correspondence) type. The point-to-point access will be hereinafter referred to as “P-P”. In the optical access network using P-P architecture, an optical line terminal (OLT) 101 for P-P and an optical network unit (ONU) 102 are connected to each other in one-to-one correspondence through an optical fiber 103. The Ethernet (registered trademark) format defined in the 802.3 committee of IEEE (Institute of Electrical and Electronics Engineers) is used as a signal format, for example.
FIG. 1B shows an example of the construction of an optical access network of a point-to-multipoint access (one-to-multiple correspondence) type called a passive optical network (PON). The point-to-multipoint access will be hereinafter referred to as “PON”. In the optical access network using PON architecture, one OLT 201 for PON and n ONUs 202-n for PON (n represents natural numbers) are connected to one another in one-to-n correspondence through an optical fiber 203, an optical splitter 235, and n optical fibers 204-n. An optical subscriber unit will be referred to as OSU and a device having plural OSUs may be referred to as OLT. In this case, the optical subscriber unit will be referred to as OLT for simplification.
In PON, upstream signals directed from the plural ONUs 202-n for PON to OLT 201 for PON are transmitted while sharing a band, so that the transmission timings thereof are not overlapping with one another when the upstream signals converge at the optical splitter 235. Such an access system is called time division multiple access (TDMA). Downstream signals directed from OLT 201 for PON to respective ONUs 202-n for PON are transmitted while information for respective ONUs for PON are multiplexed in different time frames. This communication technique is called time division multiplex (TDM). The construction and operation of PON are widely known (for example, see non-patent document 1).
Not only EPON (Ethernet Passive Optical Network) standardized as a part of Ethernet standard in IEEE 802.3, but also GPON (Gigabit capable Passive Optical Network) standardized as G.984 series in ITU-T are known for PON. The transmission speed of EPON is set to 1.25 Gbit/s for both the upstream and downstream directions. The transmission speed of GPON is set to 1.25 Gbit/s or less for upstream and to 2.5 Gbit/s or less for downstream. In addition, there are plural choices such as 622 Mbit/s, 1.25 Gbit/s, and the like.
FIG. 2 shows an example of the construction of ONU for P-P.
In FIG. 2, ONU 102 for P-P includes a bidirectional optical transceiver 111 for communicating with OLT 101 for P-P bidirectionally through the optical fiber 103 shown in FIG. 1A, a user network interface (UNI) 113 for communicating with a device on the user side, and a signal processing unit 112 for controlling one-to-one communication with OLT 101 for P-P.
ONU 102 for P-P shown in FIG. 2 performs communications by using media access control (MAC) frames defined in the Ethernet. A signal processing unit 112 has a parallelizing and decoding unit 114 for parallelizing and decoding a downstream signal received by the bidirectional optical transceiver 111, a preamble adding unit 116 for adding a predetermined preamble/SFD (start of frame delimiter) to the upstream signal, an encoding and serializing unit 115 for encoding and serializing the upstream signal added with the preamble/SFD and outputting the encoded and serialized upstream signal to the bidirectional optical transceiver 111, and a MAC functioning unit 117 for processing a transmitted and received MAC frame in accordance with header information. The preamble/SFD is a bit array of a total of eight octets added to the head of a frame to indicate the start position of the frame in the Ethernet standard, and it is defined to make up of a transmission signal, so that all the bits from the head to the seventh octet are set as idle signals “10101010 (hexadecimal form=0x55), and the eighth octet is set as SFD “10101011 (hexadecimal form=0xd5) (for example, see non-patent document 2).
The downstream signal received by ONU 102 for P-P is likewise added with a preamble, and the frame start position is specified by the parallelizing and decoding unit 114.
Furthermore, the signal processing unit 112 may be provided with OAM (operations, administration and maintenance) function unit 118 as required, so that monitoring control of a communication state and a device state of ONU can be performed from a remote place. The signal processing unit 112 is provided with a bridge/VLAN function unit 119 having a bridge function defined in IEEE 802.1D, a VLAN (virtual local area network) function defined in IEEE 802.1Q and the like, when it is necessary to convert data in accordance with a service type to be supplied or a device style to forward data transmitted from a user to a network of a telecommunication carrier (for example, see non-patent document 3).
FIG. 3 shows an example of the construction of ONU for PON.
In FIG. 3, ONU 202-n for PON includes a bidirectional optical transceiver 211 for performing bidirectional communications with OLT 201 for PON through optical fibers 203 and 204-n, and an optical splitter 235 as shown in FIG. 1B, a user network interface (UNI) 213 for connecting a device on the user side, and a signal processing unit 212 for controlling one-to-multiple communication with OLT 201 for PON.
ONU 202-n for PON shown in FIG. 3 performs one-to-multiple communications by using the function of an auxiliary layer of MPMC (multi-point MAC control) defined for EPON with P-P Ethernet set as a base. In EPON, TDMA control is implemented by MPCP (multi-point control protocol) defined in the auxiliary layer of MPMC.
The signal processing unit 212 has a parallelizing and decoding unit 214, a encoding and serializing unit 215, a preamble adding unit 216 and an MAC functioning unit 217 which operate as in the same manner as the signal processing unit 112 shown in FIG. 2, and it is also provided with an OAM function unit 218 and a bridge and VLAN function unit 219 as occasion demands.
Furthermore, the signal processing unit 212 has a preamble reading unit 222 for PON for reading a logical link number from the preamble of the downstream signal received by the parallelizing and decoding unit 214, and an MPMC function unit 220 for executing TDMA control by MPCP to perform multipoint access control.
The preamble for PON is defined in the standard of EPON so that SLD (Start of LLID Delimiter) is disposed from the head to the third octet of eight octets, a logical link identifier (LLID: Logical Link Identifier) is disposed from the sixth to seventh octets, and code error check information (CRC) of the preamble portion is disposed at the eighth octet.
SLD shows that LLID is described in a preamble area of a MAC frame of the conventional Ethernet, and the bit array thereof is defined as “10101011” (for example, see non-patent document 4). Therefore, this bit array is identical to the bit array of SFD disposed at the eighth octet of the preamble of the MAC frame for P-P.
The bidirectional optical transceiver 211 has a control unit 221 for switching an optical transmitter Tx (not shown) to an enable state (Enable) or a disable state (Disable) in accordance with an enable/disable control signal output from the MPMC function unit 220, and it is controlled to the enable state during only a time slot allocated to ONU 202-n for PON thereof.
In general, the optical access network using P-P architecture and the optical access network using PON architecture are frequently intermingled because each application area thereof such as user density, a geographical condition or the like is different.
Non-patent document 1: “GE-PON technique First, What is PON?” of Technical Fundamental Course, NTT Access Service System Research Laboratory, NTT technical Journal, Vol. 17, No. 8, August in 2005, pp. 71-74
Non-patent document 2: under the editionship of Osamu Ishida and Koichiro Seto, “Revised Edition of 10 gigabit Ethernet Text Book”, Impress Company, April in 2005, pp. 53-54
Non-patent document 3: Tsutomu Tatsuta, Noriyuki Oota, Noriki Miki and Kiyomi Kumozaki, “Design philosophy and performance of a GE-PON system for mass deployment”, JOURNAL OF OPTICAL NETWORKING, Vol. 6, No. 6, June 2007
Non-patent document 4: IEEE Standard 802.3 (version of 2005) Table 65-1-Preamble/SFD replacement mapping